There are many different types of absorbent articles for absorption, retention and isolation of body wastes such as urine, faeces and blood. Some of these known absorbent articles comprise a sensor for the detection of a soiling event, such as urination or defecation, on contact with a surface of the absorbent article or on absorption into the absorbent article. Such a sensor may, for example, be based on the detection of wetness or a biological or chemical substance in body waste, body exudates or the user's skin. When such a sensor detects a soiling event, a signal is generated, by means of which a user or caregiver, such as a parent or nursing personnel, may be made aware that a soiling event has occurred and that the absorbent article should be changed.
It is known to utilize a radiofrequency (RF) tag including an inductor-capacitor resonator as a sensor for the detection of a soiling event in an absorbent article. For example, this is described in U.S. Pat. No. 6,774,800. The RF tag may be utilized in an energy absorption mode or an energy radiation mode. In the energy absorption mode, the RF tag selectively absorbs energy from an excitation signal at its resonant frequency. This absorption produces a unique change in the excitation signal, which can be detected. When discharged fluid, such as urine, contacts the inductor-capacitor resonator of the RF tag, electrolytes in the discharged fluid create low resistance paths which detune the RF tag and thereby change its resonant frequency. A change of the resonant frequency may therefore be utilized to detect discharged fluid.
In the energy radiation mode the RF tag starts to oscillate at its resonant frequency in response to receiving an excitation signal. After termination of the excitation signal, energy stored in the RF tag causes the RF tag to continue oscillating at its resonant frequency. A response signal is thereby generated. In the presence of discharged fluid, the resonant frequency of the response signal is changed. A change of the resonant frequency of the response signal may therefore be utilized to detect discharged fluid.
In addition, it is also known to utilize a radio frequency identification (RFID) tag as a sensor for the detection of a soiling event in an absorbent article. An RFID tag that operates in the energy absorption mode absorbs energy from an excitation signal in one or more unique frequencies or unique bands of frequencies. When discharged fluid contacts said RFID tag, the fluid attenuates the excitation signal received by such an RFID tag whereby the selective absorption of energy by the RFID tag is reduced. Discharged fluid may therefore be detected.
U.S. Pat. No. 6,774,800 describes that in addition to the use of RFID tags as sensors for detection of a soiling event in absorbent articles, RFID tags that produce unique frequencies or bands of frequencies in either the energy absorption mode or energy radiation mode of operation can also be utilized for user identification. Each user of such an absorbent article in a location where there is a plurality of such users, such as a hospital or nursing home, may therefore be associated with particular frequencies or bands of frequencies produced by RFID tags that are different from the frequencies or bands of frequencies associated with all other users. Each user is then provided only with absorbent articles having RFID tags including the unique frequencies or bands of frequencies specifically assigned to him/her. However, such a system involves the risk that a specific user may be provided with the wrong absorbent article, i.e. an absorbent article having an RFID tag including frequencies or bands of frequencies that is assigned to another user. Such a mix-up may lead to erroneous data being stored in medical records and to an erroneous decision concerning when a particular absorbent article needs to be exchanged.
Another type of sensor that is utilized in some absorbent articles is the magneto-elastic sensor (which has been described by Grimes et al. in Biomedical Micro-devices, 2:51-60, 1999). A magneto-elastic sensor usually comprises a strip of magneto-elastic material. When excited by an external magnetic field, a magneto-elastic material stores magnetic energy in a magneto-elastic mode. When the magnetic field is switched off, the magneto-elastic material oscillates with a specific frequency called the magneto-acoustic resonant frequency. These oscillations give rise to a magnetic flux that varies in time, which can be read by a reader, such as a pick-up coil.
The reading of such a magneto-elastic sensor may be associated with the identity of a user of an absorbent article comprising such a sensor by providing the user with a machine readable identity tag and reading the sensor and the identity tag at the same time. Information concerning the identified user of the absorbent article may, in this way be collected and stored. Such a system avoids the risk of any mix-up occurring wherein a user is provided with an absorbent article that is intended for another user. When using such a system it must however be ensured that no other sensors are located in the vicinity of the user whose sensor reading is being taken since this may interfere with his/her reading and again lead to erroneous data being stored in medical records and to an erroneous decision of when a particular absorbent article has to be exchanged.